Urea manufacture



Patented Sept. 13, 1938 UNITED STATES PATENT OFFIQE UREA MANUFACTURE No Drawing. Application December 4, 1936, Serial No. 114,203

14 Claims.

This invention relates to preventing of corrosion and more especially to prevention of corrosion in the synthesis and handling of urea. This application is a continuation in part of my copending application Serial No. 552,795.

It is well known that when ammonia and carbon dioxide or compounds of these substances with or without water are heated under pressure, a partial conversion to urea takes place. The reaction mixtures which, for convenience, will be referred to as urea synthesis melts may vary widely in composition with varying proportions of reactants and/or with the extent to which the conversion to urea has taken place.

It is also known that urea synthesis melts during the progress of the urea reaction proper, are extremely corrosive to metals or alloys which are otherwise most suitable for urea synthesis apparatus and equipment. Similarly, when such urea synthesis melts are processed for addition thereto or removal therefrom of ingredients such as ammonia, water, fertilizing salts and the like, the melts are found to be undesirably corrosive, even at ordinary temperatures, toward the metallic apparatus and equipment utilized.

The noble metals and certain ferrous alloys containing relatively large amounts of silicon have been found to be resistant to this form of corrosion, but the usefulness of these resistant metals or alloys in urea synthesis apparatus is limited either by reason of their cost, poor machining qualities or relative weakness under stress as the case may be.

Alloy steels containing chromium with or without other major alloying constituents are available in a wide range of compositions and properties. From the standpoint of availability, not too high a cost and general physical properties these alloys are admirably adapted for use as materials of construction in reaction apparatus, stills, conduits, storage vessels and other equipment for handling urea synthesis melts. Unfortunately, however, while they are resistant to many forms of chemical attack when suitably heat treated, they are subject to rapid corrosion by urea synthesis melts. For example, the well-known alloy steel containing 18% chromium and 8% nickel, which is highly resistant to corrosion in many media is rapidly destroyed in urea synthesis melts under the temperature conditions met in urea synthesis and speedily attacked and corroded at the lower temperatures at which urea synthesis melts may be processed after the synthesis proper.

It is the object of the present invention to pro vide a method for the protection of chromium alloy steels against corrosion by urea synthesis melts.

It is a further object of this invention to provide a method for the protection of chromium alloy steel apparatus utilized in handling and processing urea synthesis melts at lower temperatures than those utilized in the synthesis of urea.

Other objects and advantages of the invention will be apparent as it is better understood by reference to the following specification in which its details and preferred embodiments are described.

I have discovered that the corrosion of chromium containing alloy steels by u rea synthesis melts is greatly reduced by dissolvingii polyvalent metal in the melt. For, although alloys of this type, even when modified by the inclusion of a polyvalent metal therein, are so rapidly attacked by urea melts as ordinarily to preclude their use in the urea synthesis, if there be added to the urea melt itself a polyvalent metal, such, for example, as copper, bismuth, cobalt, iron vanadium, mercury, chromium, nickel, zirconium, molybdenum, cerium, lead uranium, or the like, little attack upon the metal occurs even over long periods of use.

The amount of polyvalent metal required is small, although large amounts may be used without deleterious effect, the minimum amount nec-' essary to afford protection from corrosion varying with the temperature at which the urea synthesis melt is in contact with the metal and also varying somewhat with the composition of the alloy as well as with the particular polyvalent metal used. Thus, for example, copper, bismuth, cobalt, iron, vanadium, or mercury to the extent of less than 0.1% by weight of the melt have proved effective at urea-forming temperatures in the protection of chromium steels containing 16% chromium or more as their major alloying con-- stituent. Steel containing 12-14% chromium as its major alloying constituent is also protected, but requires a somewhat higher concentration of polyvalent metal in the melt. For example, substantially complete protection of this type of alloy has been secured at urea synthesis temperatures by using 0.3% copper in the urea conversion melt. Thus I have secured complete protection in the case of an alloy of 28% chromium by use of 0.05% bismuth, cobalt, copper, iron, or mercury and in the case of an alloy containing 18% chromium I have secured complete protection by the use of 0.05% bismuth, copper, or iron.

The present invention is also applicable to chromium alloy steels containing other major alloying constituents, such as, for example, nickel. Thus, the well-known 18% chromium, 8% nickel alloy steel has remained unattacked in contact with urea melts at urea-forming temperatures containing 0.02% copper, 0.03% bismuth or cosalt, 0.04% iron, 0.05% vanadium, 0.08% mercury, or 5% manganese.

Chromium alloy steels containing large quantities of nickel are likewise protected. For example, a steel containing 18% chromium, nickel and 2.5% silicon required the presence of only 0.04% copper in the solution for complete protection. I have observed, however, that the polyvalent metal requirement generally increases with increasing nickel content of the alloy and with decreasing chromium content.

In a similar manner protection is readily afforded to chrome steels containing other alloying constituents, such as manganese, tungsten, mo-

lybdenum, copper, etc.

While the corrosion reducing properties of polyvalent metals when present in urea melts are manifested generally with chromium alloy steels, I have found that to ensure maximum protection certain precautions should be observed in selecting the alloy composition. Thus, to attain substantially complete inhibition of corrosion at urea synthesis temperatures the chromium content should be not less than 12-14% in any case and, moreover, the inhibitor requirement is lowest and its effectiveness greatest when the carbon content of the alloy is relatively low. For example, the best results are obtainable with alloys whose carbon content is less than 0.1%. While the utility of the invention is by no means limited to.

alloys of the above type, I have observed generally that an increase in the carbon content of the alloy requires an increase in the inhibitor concentration.

The manner and form in which the polyvalent metal is added to the melt may be varied, it being sufficient for the purpose of the invention that a polyvalent metal be dissolved in the melt. Accordingly, the polyvalent metal may be added or obtained in the solution in any way which does not in itself involve the addition of a harmful constituent, as, for example, by dissolving in the melt at polyvalent metal as such or as a suitable compound. While, in the case of copper I prefer to inject, by pumps or other means, a solution of cupric carbonate in aqua ammonia, other sources of copper or other polyvalent metal ion are also satisfactory. Thus, metallic copper, p eferably in the presence of small amounts of cagge 1, is readily dissolved in the melt. Also, other compounds of polyvalent metals, e. g., copper oxide, bismuth nitrate, ferric chloride, cobaltous oxalate, ammonium vanadate, mercuric oxide, manganese dioxide, or the like, may be used. It is to be understood, therefore, that where reference is made broadly to a polyvalent metal substance in the appended claims this is to be taken as including the use of the element as such or in chemical combination.

I have found further that the minimum corrosion is assured in all cases if the metal surfaces are subjected to a preliminary cleaning to free them from any adhering dirt, grease, or superficial coating of oxide. This may be effected by any suitable method such as pickling or the like. I have found, however, that a very suitable and convenient cleaning agent is the hot urea melt itself so that in its preferred embodiment the urea.

invention comprises preliminarily subjecting the metal surface to be protected to contact with hot urea synthesis melt for a short time, and then adding the polyvalent metal compound which is to reduce corrosion.

The method of protection hereinbefore described has been found to be applicable to urea synthesis melts under all conditions of temperature, reactant concentration and pressure encountered in the various steps of the synthesis of urea from ammonia and carbon dioxide, including the reaction proper, the storage and transportation of the melt (if such be necessary), and

the distillation thereof to separate the products.

Although the concentrations of polyvalent metal hereinbefore described is effective in preventing corrosion of chromium-containing steels at temperatures lower than those utilized in urea synthesis I have found that much smaller concentrations of polyvalent metal is sufficient to prevent corrosion at lower temperatures. Thus, after the urea synthesis proper, which may take place at temperatures as high as 250 C. or higher, it is frequently desirable to process the resultant melt at temperatures much lower than those encountered during the synthesis. For example, it may be desired to remove unreacted excess ammonia, water, ammonium carbamate and the like from the melt at temperatures as low as 120 C. or lower and, furthermore, it may be desirable to cool the melt to ordinary temperatures, to store the melt for later processing, or the like. Under such conditions it is found that the melts, processed or not, still have corrosive characteristics even at these lower temperatures. I have found, however, that the addition of almost infinitesimally small proportions of polyvalent metal will effectively inhibit the corrosion of chromium-containing steels. Thus, concentrations of as low as 0.00015 to 0.0006% copper, based upon the weight of the melt, will effectively inhibit corrosion of chromium-containing steels at low temperatures, i. e. from temperatures ranging downward from those utilized in synthesizing For example, I have found that at temperatures of from about 40-l30 C., the concentrations of copper above described, 1. e, 0.00015 to 0.0006%, will effectively inhibit corrosion of chromium-containing steels by urea synthesis melts.

Other polyvalent metals than copper are likewise effective. Thus, at lower than urea-forming temperatures concentrations of the following polyvalent metals will be found effectively to inhibit corrosion of chromium-containing steels by urea synthesis melts: 0.000250.0009% ismuth or cobalt; 0.0003-0.0012% iron; 0.000 l0.00l5% vanadium; 0.0006-0.0024% mercury; or 0.0008- 0.0075% manganese.

I have further found that corrosion by ammonia-carbon dioxide-containing liquids, such,

, for example, as the ammonia-carbon dioxidewater-containing liquid resulting from distillation of unreacted ingredients from the urea synthesis melt may be similarly inhibited against corrosion of chromium-containing steel. Thus, for example, I have found that such a recirculated liquid, if there has been added thereto from 0.00015 to 0.0006% copper, or more, will be noncorrosive to chromium-containing steel, at temperatures up to 120 C. and above, whereas if the inhibitor is not present considerable corrosion will take place. Polyvalent metals generally will also be effective, in low concentrations, such,

for example, as the polyvalent metals hereinbefore outlined.

Various changes may be made in the method described without departing from the invention or sacrificing any of the advantages thereof.

I claim:

1. The method of avoiding corrosion of chromium alloy steel by corrosive liquid reactants necessary for and corrosive liquid reaction products resulting from the synthesis of urea from ammonia and carbon dioxide which comprises adding to such liquids a polyvalent metal selected from the group consisting of copper, bismuth, cobalt, iron, vanadium, mercury, and manganese, the polyvalent metal being added in a concentration equal to: at least 0.00015% copper, at least 0.00025% bismuth and cobalt, respectively, at least 0,0003% iron, at least 0.000375% vanadium, at least 0.0024% mercury, and at least 0.001875% manganese.

2. The method of avoiding corrosion of chromium alloy steel by corrosive liquid reaction products resulting from the synthesis of urea from ammonia and carbon dioxide which comprises adding to such liquids a polyvalent metal selected from the group consisting of copper, bismuth, cobalt, iron, vanadium, mercury, and manganese, the polyvalent metal being added in a concentration equal to at least 0.00015% copper, at least 0.00025% bismuth and cobalt, respectively, at least 0.0003% iron, at least 0.000375% vanadium, at least 0.0024% mercury, and at least 0.001875% manganese.

3. The method of avoiding corrosion of chromium alloy steel by corrosive liquid reactants necessary for the synthesis of urea from ammonia and carbon dioxide which comprises adding to such liquids a polyvalent metal selected from the group consisting of copper, bismuth, cobalt, iron, vanadium, mercury and manganese, the polyvalent metal being added in a concentration equal to at least 0.00015% copper, at least 0.00025% bismuth and cobalt, respectively, at least 0.0003% iron, at least 0.000375% vanadium, at least 0.0024% mercury, and at least 0.000l875% manganese.

4. The method of avoiding corrosion of chr0- mium alloy steel by corrosive liquid reactants necessary for and corrosive liquid reaction products resulting from the synthesis of urea from ammonia and carbon dioxide which comprises maintaining in such liquids a polyvalent metal selected from the group consisting of copper, bismuth, cobalt, iron, vanadium, mercury and manganese, the polyvalent metal being present in a concentration equal to at least 0.00015% copper, at least 0.00025% bismuth and cobalt, respectively, at least 0,0003% iron, at least 0.000375% vanadium, at least 0,0024% mercury, and at least 0.001875% manganese.

5. The method of avoiding corrosion of chromium alloy steel by corrosive liquid reaction products resulting from the synthesis of urea from ammonia and carbon dioxide which comprises maintaining in such liquids a polyvalent metal selected from the group consisting of copper, bismuth, cobalt, iron, vanadium, mercury and manganese, the polyvalent metal being present in a concentration equal to at least 0.00015% copper, at least 0.00025% bismuth and cobalt, respectively, at least 0.0003% iron, at least 0.000375% vanadium, at least 0.0024% mercury, and at least 0.001875% manganese.

6. The method of avoiding corrosion of chromium alloy steel by corrosive liquid reactants necessary for the synthesis of urea from ammonia and carbon dioxide which comprises maintaining in such liquids a polyvalent metal selected from the group consisting of copper, bismuth, cobalt, iron, vanadium, mercury and manganese, and polyvalent metal being present in a concentration equal to at least 0.000123% copper, at least 0.00025% bismuth and cobalt, respectively, at least 0.0003% iron, at least 0.000375% vanadium, at least 0.0024% mercury, and at least 0.001875% manganese.

7. The method of avoiding corrosion of chromium alloy steel by corrosive liquid reactants necessary for and corrosive liquid reaction products resulting from the synthesis of urea from ammonia and carbon dioxide which comprises adding to such liquids at least 0.00015% copper.

8. The method of avoiding corrosion of chm.- mium alloy steel by corrosive liquid reactants necessary for the synethesis of urea from ammonia and carbon'dioxide which comprises adding tov such liquids at least 0.00015% copper.

9. The method of avoiding corrosion of chromium alloy steel by corrosive liquid reaction products resulting from the synthesis of urea from ammonia and carbon dioxide which comprises adding to such liquids at least 0.00015% copper.

10. The method of avoiding corrosion by ureacontaining solutions which comprises maintaining in such solutions a polyvalent metal selected from the group consisting of copper, bismuth, cobalt, iron, vanadium, mercury, and manganese, the metal being present in a concentration equal to at least 0.02% copper, at least 0.03% bismuth, at least 0.03% cobalt, at least 0.04% iron, at least 0.05% vanadium, at least 0.08% mercury, and at least 0.25% manganese.

11. The method of avoiding corrosion by ureacontaining solutions which comprises maintaining at least 0.02% copper in a urea synthesis melt from which at least a part of the unconverted reactants have been removed.

12. A method of avoiding apparatus corrosion in synthesizing urea in a chromium alloy steel vessel which comprises dissolving at least 0.03% cobalt in the urea synthesis melt.

13. A method of avoiding apparatus corrosion in synthesizing urea in a chromium alloy steel vessel which comprises dissolving at least 0.03% bismuth in the urea synthesis melt.

14. A method of avoiding apparatus corrosion in synthesizing urea in a chromium alloy steel vessel which comprises dissolving at least 0.02% copper in the urea synthesis melt.

HARRY C. HETHERINGTON. 

